U.S. Pat. No. 5,782,606 and French patent No. 2 742 726 describe a blade-folding device in which the root of the blade is secured to a folding fitting having two rotary bearings enabling the blade to pivot relative to the end of a sleeve providing a mechanical connection between the blade and the rotor hub. The rotor disclosed in those patents includes a plate pierced by as many cells as it has blades. Each sleeve for securing a blade to the hub comprises two arms forming a fork and is secured to the hub via a laminated spherical abutment secured to the arms of the sleeve, said resilient abutment enabling the sleeve—and the corresponding blade—to be hinged relative to the rotor hub about three pivot axes:
i) a hinge about a first axis that is substantially parallel to the longitudinal axis of the rotor shaft to enable the blade to be moved “in drag” relative to the hub;
ii) a hinge of the sleeve and the blade about a second axis that substantially coincides with the longitudinal axis of the blade to accommodate the variation in pitch (and incidence) that are required for the blade; and
iii) a hinge about a third axis that is substantially perpendicular to the longitudinal axis of the rotor shaft, to enable the blade to perform flapping movements relative to the rotor hub.
In particular, when the hinge between the hub and the blade (and sleeve) is provided via a spherical abutment, these three hinge axes coincide at the geometrical center of the abutment.
The presence of these three degrees of freedom in rotation for the blades relative to the hub is necessary during rotation of the rotor, and in particular in flight, but when the rotor is stationary, it constitutes an impediment against folding the blades of the rotor.
Before folding the blade it is necessary to lock these three degrees of freedom so as to prevent them producing their effects. The flapping hinge is generally held stationary by a portion of the sleeve—or a “low”abutment secured thereto—moving under the effect of the weight of the blade and the associated sleeve to come into abutment against a complementary abutment integrated in the hub or secured thereto, and generally referred to as the reciprocating stop ring by the person skilled in the art.
It remains necessary to lock the sleeve in position relative to the hub about the drag and pitch hinge axes.
To this end, a locking device can be provided for a hinged holding structure (such as a sleeve) for securing a blade of a rotorcraft rotor to a hub of the rotor, which device comprises a lever arranged to cause said holding structure to pivot about the drag axis into a drag locking position in which said structure bears against a “drag” abutment secured to the hub. The lever can simultaneously cause the blade-holding structure (sleeve) to pivot about the pitch axis into a pitch locking position in which the structure bears against a “pitch” abutment connected to the hub.
Prior to folding the blades of the rotor, the lever can be operated, i.e. caused to move, by an electric actuator, such as a rotary motor, so as to act under the control of a unit for controlling the locking of the sleeve, to cause the position of the blade-holding structure to be locked automatically relative to the hub.
Such a locking device is described in particular in U.S. Pat. No. 5,951,251.